The present invention relates generally to oxygen measurement devices, and more specifically to an oxygen sensor for aircraft fuel tanks.
Aircraft air/fuel mixtures are explosive at oxygen concentrations above approximately 12 percent, therefore it is desirable to monitor and control the oxygen concentration in fuel tanks in order to avoid explosions. These explosions may be due to lightning strikes, electrical shorts, static electricity and, in the case of military aircraft, hostile combat environments. Although some fuels are less volatile, additional fuel tank protection is still desirable. One approach to obtaining fuel tank protection is through fuel tank inerting.
There are two basic techniques in use or under development for modern aircraft fuel tank inerting; first, on-board liquid nitrogen systems and second, on-board inert gas generation systems (OBIGGS). The former approach is cost prohibitive for many applications, primarily due to the extensive ground support required (e.g., special nitrogen servicing trucks and liquid nitrogen storage tanks). The primary concerns with regards to self-generating systems are the cost and weight aspects as well as their ability to supply sufficient inert gas to repressurize the fuel tanks during each descent.
In either of the fuel tank inerting systems described above it is desirable to monitor the oxygen content of the fuel tank and the inerting system supply gas, thus indicating both the relative flammability of the fuel vapor mixture and the effectiveness of the inerting system. The extremely hostile environment of the aircraft as well as the need for high accuracy, stability, and freedom from interference make the use of electro-chemical sensors impractical. Furthermore, a suitable oxygen monitor must be able to both survive and correct for the rapid and extreme pressure and temperature changes common to aircraft flights.
Tell et al. (U.S. Pat. No. 5,173,749) describe a method and apparatus for making spectroscopic measurements of the concentration of a gas in a sample cell using a modulated laser diode as the light source. Applicability of this apparatus to both medical and industrial applications is described. The '749 patent does not disclose how to correct for or measure either temperature or pressure using a single spectroscopic instrument.